Switching to Login on its own

Hi All, when running my MacBook Pro, (2.3 GHz Intel Core i7, 8GB 1600 MHz DDR3, Mac OSX V10.7.5) it sometimes switches to the login screen all on its own, when I log back in all apps startup again but from a closed state and I may loose any changes made to documents etc, iTunes will not start where it left off etc. I am typically running 5 or 6 applications say email, iTunes, iPhoto, Safari, Skype and sometimes others, I may have my iPod or iPhone plugged in or syncing but there doesn’t seem to be any particular combination or reason. Does anyone else have this or have you managed to resolve it?